n = input() 
 
if len(n) != 4 or not n.isdigit(): 
    print(0) 
else: 
    max_prime = 0 
    for i in range(4): 
        for j in range(i + 1, 5): 
            num = int(n[i:j]) 
            is_prime = True 
            if num <= 1: 
                is_prime = False 
            else: 
                for k in range(2, int(num**0.5) + 1): 
                    if num % k == 0: 
                        is_prime = False 
                        break 
            if is_prime and num > max_prime: 
                max_prime = num 
    print(max_prime)